Lithium secondary battery and non-aqueous electrolytic solution

ABSTRACT

The sudden generation of heat being frequently caused in the case of the overcharge of a lithium secondary cell which have a positive electrode comprising a composite metal oxide of lithium and cobalt or a composite metal oxide of lithium and nickel, a negative electrode comprising metallic lithium, a lithium alloy or a material capable of occluding and releasing lithium, and a nonaqueous electrolyte solution comprising a nonaqueous solvent and an electrolyte dissolved therein can be efficiently prevented by the addition, to the nonaqueous electrolyte solution, of an organic compound which, when the lithium secondary cell is overcharged, decomposes into a decomposition product capable of dissolving out the cobalt or nickel contained in the positive electrode and depositing it ion the negative electrode (for example, a tert-alkylbenzene derivative).

FIELD OF INVENTION

The present invention relates to a lithium secondary battery and anon-aqueous electrolytic solution which is favorably employable for thelithium secondary battery. The invention specifically relates to alithium secondary battery improved in battery characteristics such ascycle performance, battery capacity and storage performance and furtherin its safety such as prevention of sudden heat generation which iscaused in the case of overcharging, and a non-aqueous electrolyticsolution which is favorably employable for the lithium secondarybattery.

BACKGROUND OF INVENTION

At present, a lithium secondary battery is generally employed as anelectric source for driving small electronic devices. The lithiumsecondary battery is expected not only for the use as a potableelectronic/communication tool such as small size video camera, potablephone, and note-size personal computer but also an electric source ofautomobile. The lithium secondary battery essentially comprises apositive electrode, a non-aqueous electrolyte solution, separator, and anegative electrode. A lithium secondary battery utilizing a positiveelectrode of lithium compound oxide such as LiCoO₂ and a negativeelectrode of carbonaceous material or lithium metal is generally used.As the electrolyte solution, a carbonate such as ethylene carbonate (EC)or propylene carbonate (PC) is generally used.

When the lithium secondary battery is overcharged to a level higher thanthe ordinary working voltage, an excessive amount of lithium is releasedfrom the positive electrode, and simultaneously excessive lithiumdeposits on the negative electrode, and dendrite is produced. Therefore,both of the positive electrode and the negative electrode are renderedchemically unstable. If both of the positive and negative electrodesbecome chemically unstable, they soon react with carbonate in thenon-aqueous electrolyte solution to decompose the carbonate, and suddenexothermic reaction occurs. Accordingly, the battery as such generatesabnormal heat, and trouble of lowering of battery safety occurs. Thetrouble will be more serious in the case that the energy density of alithium secondary battery increases.

Japanese Patent Provisional Publication 7-302614, for instance, proposesthat a small amount of an aromatic compound be added to the electrolytesolution so that the safety to the overcharging can be ensured and theabove-described trouble can be obviated. The Japanese Patent ProvisionalPublication 7-302614 describes anisole derivatives that have a molecularweight of not more than 500 and a π-electron orbital showing areversible oxidation-reduction potential at a potential of more noblethan the positive electrode potential in the case of full charging. Itis explained that the anisole derivative functions as a redox shuttle inthe battery so as to ensure safety of battery when the battery isovercharged.

Japanese Patent Provisional Publication 9-106835 discloses a method forensuring safety of a battery under overcharging condition by employingcarbonaceous material as the negative electrode and incorporatingapprox. 1 to 4% of an additive such as biphenyl, 3-R-thiophene,3-chlorothiophene or furan into the electrolyte solution so thatbiphenyl or the like produces a polymer to enhance internal resistanceof the battery when the voltage of the battery exceeds the predeterminedmaximum working voltage.

Japanese Patent Provisional Publication 9-171840 discloses a method forensuring safety of a battery under overcharging condition by similarlyemploying biphenyl, 3-R-thiophene, 3-chlorothiophene or furan whichpolymerizes to produce gaseous material so as to initiate the internalcurrent-disconnecting apparatus for forming internal short-circuit whenthe voltage of the battery exceeds the predetermined maximum workingvoltage.

Japanese Patent Provisional Publication 10-321258 discloses a method forensuring safety of a battery under overcharging condition by similarlyemploying biphenyl, 3-R-thiophene, 3-chlorothiophene or furan whichpolymerizes to produce an electro-conductive polymer for forminginternal short-circuit when the voltage of the battery exceeds thepredetermined maximum working voltage.

Japanese Patent Provisional Publication 11-162512 points out a problemin the use of an additive such as biphenyl or the like in a battery inthat the battery characteristics such as cycle characteristic are apt tolower when the cyclic procedure is repeated up to a voltage exceeding4.1 V or the battery is discharged at a high temperature exceeding 40°C. for a long period of time, and that this problem is more prominentlyobserved when the addition amount of additive increases. For ensuring abattery under over-charging condition, this publication then proposes anelectrolyte solution in which 2,2-diphenylpropane or other additive isincorporated and the 2,2-diphenylpropane or the like polymerizes toproduce a gaseous material to initiate the internalcurrent-disconnecting apparatus or give an electro-conductive polymerfor forming the internal short-circuit when the voltage of the batteryexceeds the predetermined maximum working voltage.

Although the anisole derivative disclosed in Japanese Patent ProvisionalPublication 7-302614 favorably functions by redox shuttle in the case ofovercharging, it has problems in that adverse effects are observed oncycle characteristics and storage stability. In more detail, the anisolederivative described in the publication gradually decomposes in thecharge-discharge procedure when the battery is employed at a hightemperature such as higher than 40° C. or subjected locally to arelatively high voltage in the use at an ordinary working voltage.Therefore, the battery characteristics lower. Thus, the amount of ananisole derivative gradually decreases in the course of ordinarycharge-discharge procedures, and hence the safety may not be ensuredafter the charge-discharge procedures of 300 cycles.

The biphenyl, 3-R-thiophene, 3-chlorothiophene, and furan described inJapanese Patent Provisional Publications 9-106835, 9-171840, and10-321258 also favorably work when overcharging occurs. However, as ispointed out in the aforementioned Japanese Patent ProvisionalPublication 11-162512, they impart adverse effect to the cyclecharacteristics and storage stability. Further, the adverse effectincreases when the amount of biphenyl and the like is increased. In moredetail, the biphenyl or the like is oxidized and decomposes at apotential of 4.5 V or less. Therefore, the biphenyl or the likedecomposes in the charge-discharge procedure when the battery isemployed at a high temperature such as higher than 40° C. or subjectedlocally to a relatively high voltage in the use at an ordinary workingvoltage. Therefore, the battery characteristics lower. Thus, the amountof a biphenyl or the like gradually decreases in the course of ordinarycharge-discharge procedures, and hence the safety may not be ensuredafter the charge-discharge procedures of 300 cycles.

The battery containing 2,2-diphenylpropane which is described inJapanese Patent Provisional Publication 11-162512 does not show such ahigh safety at the time of overcharging as the safety shown in thebattery containing biphenyl. However, it still show a high safety at thetime of overcharging, as compared with a battery containing no additive.Further, although the battery containing 2,2-diphenylpropane shows highcycle characteristics as compared with the cycle characteristics shownin the battery containing biphenyl, it still does not show such highcycle characteristics, as compared with a battery containing noadditive. Thus, the publication describes that the good cyclecharacteristics may be accomplished only when the safety is partiallyignored. In consequence, it does not satisfy either batterycharacteristics or safety such as prevention of overcharging.

The invention has an object to provide a lithium secondary battery whichare free from the above-mentioned problems, that is, which is improvedin safety such as prevention of sudden heat generation at the time ofovercharging, and other battery characteristics such as cyclecharacteristics, electric capacity and storage stability.

DISCLOSURE OF INVENTION

The present invention resides in a method of preventing sudden heatgeneration when a lithium secondary battery comprising a positiveelectrode comprising a composite metal oxide of lithium and cobalt or acomposite metal oxide of lithium and nickel, a negative electrodecomprising a lithium metal, a lithium alloy or a material capableoccluding and releasing lithium, and a non-aqueous electrolyte solutioncomprising an electrolyte in a non-aqueous solvent is overcharged, whichcomprises dissolving an organic compound in the non-aqueous electrolytesolution, decomposing the organic compound when the overcharging takesplace, to give a decomposed product, the decomposed product functioningto dissolve cobalt or nickel out of the positive electrode, and depositthe cobalt or nickel on the negative electrode.

The invention further resides in a lithium secondary battery comprisinga positive electrode comprising a composite metal oxide of lithium andcobalt or a composite metal oxide of lithium and nickel, a negativeelectrode comprising a lithium metal, a lithium alloy or a materialcapable occluding and releasing lithium, and a non-aqueous electrolytesolution comprising an electrolyte in a non-aqueous solvent, in which anorganic compound is contained in the non-aqueous electrolyte solution,said organic compound decomposing when overcharging of the secondarybattery takes place to give a decomposed product, said decomposedproduct functioning to dissolve cobalt or nickel out of the positiveelectrode and deposit the cobalt or nickel on the negative electrode.

The invention furthermore resides in a lithium secondary batterycomprising a positive electrode comprising a composite metal oxide oflithium and cobalt or a composite metal oxide of lithium and nickel, anegative electrode comprising a lithium metal, a lithium alloy or amaterial capable occluding and releasing lithium, and a non-aqueouselectrolyte solution comprising an electrolyte in a non-aqueous solvent,which further contains an organic compound having an oxidation potentialin the range of +4.6 V to +5.0 V, which is determined relatively to theoxidation potential of lithium.

The invention furthermore resides in a non-aqueous electrolyte solutionto be used for a lithium secondary battery comprising a positiveelectrode comprising a composite metal oxide of lithium and cobalt or acomposite metal oxide of lithium and nickel, a negative electrodecomprising a lithium metal a lithium alloy or a material capableoccluding and releasing lithium, and a non-aqueous electrolyte solutioncomprising an electrolyte in a non-aqueous solvent, which furthercontains an organic compound having an oxidation potential in the rangeof +4.6 V to +5.0 V, which is determined relatively to the oxidationpotential of lithium.

As described hereinbefore, known methods for preventing sudden heatgeneration caused by overcharging (i.e., thermal runaway) so as toensure safety of the battery are as follows: the method of functioningredox shuttle at a potential of approx. 4.5 V; the method of increasingthe internal resistance by producing a polymer at a potential of 4.5 Vor lower; and the method of forming a short circuit in the battery byproducing a gaseous material to initiate an internalcurrent-disconnecting device or by producing an electro-conductivepolymer.

In contrast, the mechanism of the prevention of overcharging accordingto the invention is considered as follows:

When the battery is overcharged, the above-mentioned compound containedin the non-aqueous electrolyte solution oxidatively decomposes at apotential in the range of +4.6 V to +5.0 V, as compared with thelithium, and cobalt or nickel in the positive electrode dissolves outand deposits on the negative electrode. It is assumed that the cobalt ornickel deposited on the negative electrode prevent reaction between thelithium metal deposited on the negative electrode and the carbonatecontained in the non-aqueous electrolyte solution.

Further, the deposition of cobalt or nickel on the negative electrode ofthe battery may sometimes cause formation of short circuit so that theprevention of overcharging can be made. Thus, the safety of battery issufficiently ensured.

Furthermore, since the aforementioned organic compound has a highoxidation potential of +4.6 V to +5.0 V, as compared with the oxidationpotential of lithium, the organic compound does not decompose at atemperature of 40° C. or higher, in the course of repeatedcharge-discharge procedures and in the case that the voltage locallyexceeds 4.2 V.

Accordingly, a lithium secondary battery having not only good safety forpreventing overcharging but also good battery characteristics such ascycle characteristics, battery capacity and storage stability isprovided.

BEST EMBODIMENTS FOR PERFORMING INVENTION

Examples of the organic compounds to be incorporated into theelectrolyte solution according to the invention include the followingcompounds. The oxidation potential of each organic compound, as comparedwith that of lithium (determined in the manner described in thebelow-stated working examples) is described in parenthesis.

As the organic compound, at least one of tert-alkylbenzene derivativesis preferably used. Examples are as follows: tert-butylbenzenederivatives such as tert-butylbenzene (4.9 V),1-fluoro-4-tert-butylbenzene (4.9 V), 1-chloro-4-tert-butylbenzene (4.9V), 1-bromo-4-tert-butylbenzene (4.9 V), 1-iodo-4-tert-butylbenzene (4.9V), 5-tert-butyl-m-xylene (4.6 V), 4-tert-butyltoluene (4.7 V),3,5-di-tert-butyltoluene (4.8 V), 1,3-di-tert-butyl-benzene (4.9 V),1,4-di-tert-butylbenzene (4.9 V), 1,3,5-tri-tert-butylbenzene (5.0 V);and tert-alkylbenzene derivatives such as tert-pentylbenzene (4.8 V),1-methyl-4-tert-pentylbenzene (4.7 V), 5-tert-pentyl-m-xylene (4.6 V),1-ethyl-1-(methylpropyl) benzene (4.8 V), (1,1-di-ethylpropyl)benzene(4.8 V), 1,3-di-tert-pentylbenzene (4.7 V), and1,4-di-tert-pentylbenzene (4.7 V).

As the organic compound, cyclohexylbenzene (4.7 V) is also employed.Particularly, if a portion of the organic compound having a highoxidation potential such as within 4.8-5.0 V such as the above-mentionedtert-butylbenzene is replaced with cyclohexylbenzene having a lowoxidation potential such as 4.7 V, the prevention of overcharging isenhanced. In the case that a portion of the tert-butylbenzene isreplaced with cyclohexylbenzene, the tert-butylbenzene is preferablyemployed in an amount of 4 or less parts, more preferably 0.3 to 3parts, most preferably 0.5 to 2.5 parts, per one part ofcyclohexylbenzene. As is described above, the use of combination of twoor more organic compounds having different oxidation potentials iseffective to enhance the prevention of overcharging. However, thecompound of the present invention is not restricted by these compounds,so long as the organic compounds decompose at a potential in the rangeof +4.6 V to +5.0 V and the cobalt or nickel in the positive electrodedissolves in the solution at the time of overcharging.

If the amount of the organic compound is excessively large, the electricconductivity of the electrolyte solution may vary and hence the batterycharacteristics may lower. If the amount is too small, enough effect toprevent sudden heat generation caused by overcharging is attained. Theamount preferably is in the range of 0.1 to 10 weight %, more preferably1 to 5 weight %, per the amount of the electrolyte solution.

Examples of the non-aqueous solvents employed in the invention includecyclic carbonates such as ethylene carbonate (EC), propylene carbonate(PC), butylene carbonate (BC), and vinylene carbonate (VC); lactonessuch as γ-butylolactone; linear carbonates such as dimethyl carbonate(EMC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC); etherssuch as tetrahydrofuran, 2-methyl-tetrahydrofuran, 1,4-dioxane,1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane;nitriles such as acetonitrile; esters such as methyl propionate, methylpivalate and octyl pivalate; and amides such as dimethylformamide.

The non-aqueous solvents can be employed singly or in combination of twoor more. There are no limitations with respect to the combinations.Examples are a combination of a cyclic carbonate and a linear carbonate,a combination of a cyclic carbonate and a lactone, and a combination ofthree cyclic carbonates and a linear carbonate.

Examples of the electrolytes include LiPF₆, LiBF₄, LiClO₄, LiN(SO₂CF₃)₂,LiN(SO₂C₂F₅)₂, LiC(SO₂CF₃)₃, LiPF₄(CF₃)₂, LiPF₃(C₂F₅)₃, LiPF₃(CF₃)₃,LiPF₃(iso-C₃F₇)₃, and LiPF₅(iso-C₃F₇). These electrolytes can beemployed singly or in combination of two or more. Generally, theelectrolyte can be incorporated into the non-aqueous solvent in such anamount to give an electrolyte solution of 0.1 M to 3 M, preferably 0.5 Mto 1.5 M.

The electrolyte solution can be prepared, for instance, by mixing theabove-mentioned non-aqueous solvents and dissolving at least one organiccompound in the mixed solvents.

The electrolyte solution of the invention is favorably employable as anelement for manufacture of a secondary battery, particularly a lithiumsecondary battery. There are no specific restrictions with respect toother constitutional elements. Various heretofore-employed elements canbe used.

For instance, compound metal oxides comprising lithium and cobalt ornickel are employable as the positive electrode active materials.Examples of the compound metal oxides include LiCoO₂, LiNiCO₂, andLiCo_(1-x)Ni_(x)O₂ (0.01<x<1). Further, a mixture of LiCoO₂ and LiMn₂O₄,a mixture of LiCoO₂ and LiNiO₂, and a mixture of LiMn₂O₄ and LiNiO₂ canbe employed.

The positive electrode can be manufactured by kneading theabove-mentioned positive active material, an electro-conductive materialsuch as acetylene black or carbon black, and a binder such aspoly(tetrafluoroethylene) (PTFE), poly(vinylidene fluoride) (PVDF),styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer(NBR), or carboxymethylcellulose (CMC) to give a positive electrodecomposition; coating the positive electrode composition on a collectorsuch as aluminum foil, stainless foil, or lath plate; and heating thecoated composition in vacuo at a temperature of approximately 50 to 250°C. for approximately 2 hours.

As the negative electrode active material, a lithium metal, lithiumalloys, carbonaceous materials capable of occluding and releasinglithium (thermally decomposed carbons, cokes, graphites (artificialgraphite and natural graphite), fired organic polymer compounds,carbonaceous fibers), and compound tin oxides can be employed. It ispreferred to employ carbonaceous materials having a graphite crystalstructure in which the lattice distance of lattice surface (002),namely, d₀₀₂, is in the range of 0.335 to 0.340 nm (nanometer). Thenegative electrode active material in the powdery form such ascarbonaceous powder is preferably used in combination with a binder suchas ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene(PTFE), poly(vinylidene fluoride) (PVDF), styrene-butadiene copolymer(SBR), acrylonitrile-butadiene copolymer (NBR) or carboxymethylcellulose(CMC).

There are no specific limitations with respect to the structure of thenon-aqueous secondary battery. For instance, the non-aqueous secondarybattery can be a battery of coin type or a polymer battery comprising apositive electrode, a negative electrode, and single or pluralseparators, or a cylindrical or prismatic battery comprising a positiveelectrode, a negative electrode, and a separator roll. The separatorscan be known separators such as micro-porous separators of polyolefin,other micro-porous films, woven fabrics and non-woven fabrics.

The lithium secondary battery of the invention shows excellent cyclecharacteristics for a long period of time even at a high working voltagesuch as a voltage higher than 4.2 V, and further shows excellent cyclecharacteristics even at 4.3 V. The cut-off voltage can be 2.0 V orhigher and moreover can be 2.5 V or higher. The current value is notlimited. Generally, the battery works at a constant current discharge of0.1 to 3 C. The lithium secondary battery of the invention can becharged and discharged in the broad temperature range of −40° C. to 100°C., preferably within 0 to 80°C.

The present invention is further described by the following examples andcomparison examples.

EXAMPLE 1 Measurement of Oxidation Potential

LiPF₆ was dissolved in propylene carbonate (non-aqueous solvent) to givean electrolyte solution (concentration: 1 M). In the electrolytesolution was dissolved tert-butylbenzene in an amount of 2 weight %. Theoxidation potential was measured at room temperature (20° C.) by meansof an electrochemical analyzer (Model 608A, available from ALSCorporation). The reference electrode was a lithium metal, and theworking electrode was a platinum pole electrode (diameter 1 mm). Thescanning was made from +3 V to +6 V at a rate of 10 mV/sec. Thepotential value at which a current variation of 0.1 mA was observed wasdetermined to be the oxidation potential. The measured value was roundedto two decimal places. As a result, the oxidation potential oftert-butylbenzene was determined to be 4.9 v.

(Preparation of Electrolyte Solution)

A non-aqueous mixture of EC/PC/DEC (30/5/65, volume ratio) was prepared.In the aqueous mixture was dissolved LiPF₆ at a concentration of 1 M.Further, tert-butylbenzene was placed in the electrolyte solution at aconcentration of 2 weight %.

(Manufacture of Lithium Secondary Battery and Measurements of BatteryCharacteristics)

LiCoO₂ (positive electrode active material, 90 wt. %), acetylene black(electro-conductive material, 5 wt. %), and poly(vinylidene fluoride)(binder, 5 wt. %) were mixed. To the resulting mixture was added1-methyl-2-pyrrolidone. Thus produced mixture in the form of slurry wascoated on aluminum foil, dried, and pressed to give a positiveelectrode.

Artificial graphite (negative electrode active material, 95 wt. %) andpoly(vinylidene fluoride) (binder, 5 wt. %) were mixed. To the resultingmixture was further added 1-methyl-2-pyrrolidone. Thus produced mixturein the form of slurry was coated on copper foil, dried, and pressed togive a negative electrode.

The positive and negative electrodes, a micro-porous polypropylene filmseparator, and the above-mentioned non-aqueous electrolytic solutionwere combined to manufacture a cylindrical battery (18650 size,diameter: 18 mm, thickness: 65 mm). The battery were equipped with apressure-releasable opening and an internal current-disconnectingdevice.

The 18650 battery was charged at a high temperature (45° C.) with aconstant electric current (1.45 A, 1C) under constant voltage to reach4.2 V and further charged to the terminal voltage 4.2 V. The totalperiod of time was 3 hours. Subsequently, the battery was discharged togive a constant electric current (1.45 A, 1C) to give a terminal voltageof 2.5 V. The charge-discharge cycle was repeated.

The initial discharge capacity was almost the same as the capacitymeasured in a battery using an 1M LiPF₄ and an EC/PC/DEC (30/5/65,volume ratio) solvent mixture (see Comparison Example 1).

After the 300 cycle charge-discharge procedures, the retention ofdischarge capacity was 85.5% of the initial discharge capacity (100%).The high temperature storage stability was also good. The 18650 batteryhaving been subjected to the 300 cycle charge-discharge procedures wasthen overcharged by continuously charging the full charged batteryfurther at a room temperature (20° C.) under a constant current (2.9 A,2C). The current was disconnected after 25 minutes, and the highestsurface temperature of the battery after the current disconnection was68° C.

The materials and conditions of the 18650 size cylindrical battery andthe battery characteristics are set forth in Table 1.

EXAMPLE 2

The procedures of Example 1 were repeated except that tert-butylbenzenewas added to the electrolyte solution in an amount of 5 weight %, tomeasure the oxidation potential. The result is set forth in Table 1.

The materials and conditions of the 18650 size cylindrical battery aswell as the discharge capacity retention after the 300 cyclecharge-discharge procedures, the current disconnection period, and thehighest surface temperature of the battery after the currentdisconnection are set forth in Table 1.

EXAMPLE 3

The procedures of Example 1 were repeated except that each oftert-butylbenzene and cyclohexylbenzene was added to the electrolytesolution in an amount of 1 weight %, to measure the oxidationpotentials. The results are set forth in Table 1.

The materials and conditions of the 18650 size cylindrical battery aswell as the discharge capacity retention after the 300 cyclecharge-discharge procedures, the current disconnection period, and thehighest surface temperature of the battery after the currentdisconnection are set forth in Table 1.

It is understood that the temperature after current disconnection is lowand the current disconnection period is short, as compared with those,measured in Example 1. Accordingly, it is understood that the effect ofpreventing overcharging is higher than that shown in Example 1.

EXAMPLE 4

The procedures of Example 1 were repeated except that1-bromo-4-tert-butylbenzene was added to the electrolyte solution in anamount of 2 weight %, to measure the oxidation potential. The result isset forth in Table 1.

The materials and conditions of the 18650 size cylindrical battery aswell as the discharge capacity retention after the 300 cyclecharge-discharge procedures, the current disconnection period, and thehighest surface temperature of the battery after the currentdisconnection are set forth in Table 1.

COMPARISON EXAMPLE 1

The procedures of Example 1 were repeated except that tert-butylbenzenewas not added to the electrolyte solution, to measure the oxidationpotential. The result is set forth in Table 1.

The materials and conditions of the 18650 size cylindrical battery aswell as the discharge capacity retention after the 300 cyclecharge-discharge procedures, the current disconnection period, and thehighest surface temperature of the battery after the currentdisconnection are set forth in Table 1.

COMPARISON EXAMPLES 2 TO 4

The procedures of Example 1 were repeated except that, in place oftert-butylbenzene, 4-fluoroanisole (Comparison Example 2),2-chlorothiophene (Comparison Example 3) or biphenyl (Comparison Example4) was added to the electrolyte solution in an amount of 2 weight %, tomeasure the oxidation potential. The results are set forth in Table 1.

The materials and conditions of the 18650 size cylindrical battery aswell as the discharge capacity retention after the 300 cyclecharge-discharge procedures, the current disconnection period, and thehighest surface temperature of the battery after the currentdisconnection are set forth in Table 1.

EXAMPLE 5

The procedures of Example 1 were repeated except that LiCoO₂ (positiveelectrode active material) was replaced with LiNi_(0.8)CO_(0.2)O₂, anelectrolyte solution of 1 M LiPF₆ in a non-aqueous solvent ofEC/PC/VC/DEC (30/5/2/63, volume ratio) was employed, andtert-butylbenzene was added to the electrolyte solution in an amount of3 weight %, to manufacture a 18650 size cylindrical battery and measurethe battery characteristics. The materials and conditions of the 18650size cylindrical battery and the battery characteristics are set forthin Table 1.

EXAMPLE 6

The procedures of Example 1 were repeated except that LiCoO₂ (positiveelectrode active material) was replaced with LiNi_(0.8)Co_(0.2)O₂, anelectrolyte solution of 1 M LiPF₆ in a non-aqueous solvent ofEC/PC/VC/DEC (30/5/2/63, volume ratio) was employed, andtert-butylbenzene and cyclohexylbenzene were added to the electrolytesolution in amounts of 2 weight % and 1 weight %, respectively, tomanufacture a 18650 size cylindrical battery and measure the batterycharacteristics. The materials and conditions of the 18650 sizecylindrical battery and the battery characteristics are set forth inTable 1.

COMPARISON EXAMPLE 5

The procedures of Comparison Example 1 were repeated except that LiCoO₂(positive electrode active material) was replaced withLiNi_(0.8)Co_(0.2)O₂, to manufacture a 18650 size cylindrical batteryand measure the battery characteristics. The materials and conditions ofthe 18650 size cylindrical battery and the battery characteristics areset forth in Table 1.

EXAMPLE 7

The procedures of Example 1 were repeated except that tert-butylbenzenewas replaced with tert-pentylbenzene in an amount of 2 weight %, tomanufacture a 18650 size cylindrical battery and measure the batterycharacteristics. The materials and conditions of the 18650 sizecylindrical battery and the battery characteristics are set forth inTable 1.

EXAMPLE 8

The procedures of Example 1 were repeated except that tert-butylbenzenewas replaced with a combination of tert-butylbenzene andtert-pentylbenzene both in amounts of 2 weight %, to manufacture a 18650size cylindrical battery and measure the battery characteristics. Thematerials and conditions of the 18650 size cylindrical battery and thebattery characteristics are set forth in Table 1.

EXAMPLE 9

The procedures of Example 1 were repeated except that tert-pentylbenzeneand cyclohexylbenzene were added to the electrolyte solution in amountsof 2 weight % and 1 weight %, respectively, to manufacture a 18650 sizecylindrical battery and measure the battery characteristics. Thematerials and conditions of the 18650 size cylindrical battery and thebattery characteristics are set forth in Table 1.

EXAMPLE 10

The procedures of Example 1 were repeated except that tert-butylbenzene,tert-pentylbenzene and cyclohexylbenzene were added to the electrolytesolution in amounts of 2 wt. %, 2 wt. %, and 1 wt. %, respectively, tomanufacture a 18650 size cylindrical battery and measure the batterycharacteristics. The materials and conditions of the 18650 sizecylindrical battery and the battery characteristics are set forth inTable 1.

In all of the above-mentioned Examples, a sufficient amount of cobalt ornickel was deposited on the negative electrode when the overchargingtook place. Accordingly, it is clear that the battery confining theorganic compound of the invention are superior to the battery ofComparison Example in the safety in the overcharging and cyclecharacteristics.

TABLE 1 Organic Discon. Max. Retention Posi./ Compound Oxid. PeriodTemp. after 300 Nega. (wt. %) Pot. (min.) (° C.) cycle (%) Ex. 1 LiCoO₂/tert-butyl- 4.9 25 68 85.5 Art. G. benzene (2) Ex. 2 LiCoO₂/ tert-butyl-4.9 23 66 85.1 Art. G. benzene (5) Ex. 3 LiCoO₂/ tert-butyl- 4.9 18 6485.3 Art. G. benzene (1) cyclohexyl- 4.7 benzene (1) Ex. 4 LiCoO₂/1-bromo-4- 4.9 26 69 85.2 Art. G. tert-butyl- benzene (2) Com. 1 LiCoO₂/None 5.4 31 T.R. 82.8 Art. G. Com. 2 LiCoO₂/ 4-fluoro- 4.5 22 118 72.6Art. G. anisole (2) Com. 3 LiCoO₂/ 2-chloro- 4.4 19 92 73.3 Art. G.thiophene(2) Com. 4 LiCoO₂/ biphenyl 4.5 18 83 74.2 Art. G. (2) Ex. 5(comp)/ tert-butyl- 4.9 24 67 84.7 Art. G. benzene (3) Ex. 6 (comp)/tert-butyl- 4.9 19 65 84.3 Art. G. benzene (2) cyclohexyl- 4.7 benzene(1) Com. 5 (comp)/ None 5.4 31 T.R. 80.4 Art. G. Ex. 7 LiCoO₂/tert-pentyl- 4.8 22 66 85.3 Art. G. benzene (2) Ex. 8 LiCoO₂/tert-butyl- 4.9 20 64 85.2 Art. G. benzene (2) tert-pentyl- 4.8 benzene(2) Ex. 9 LiCoO₂/ tert-pentyl- 4.8 17 63 84.7 Art. G. benzene (2)cyclohexyl- 4.7 benzene (1) Ex. 10 LiCoO₂/ tert-butyl- 4.9 17 63 84.9Art. G. benzene (2) tert-pentyl- 4.8 benzene (2) cyclohexyl- 4.7 benzene(1) Remarks: Art. G. = Artificial Graphite (comp) = LiNi_(0.8)Co_(0.2)O₂Oxid. Pot. = Oxidation Potential Discon. Period = Current DisconnectionPeriod Max. Temp. = Maximum (Highest) Temperature Retention after 300cycle = Retention of Discharge Capacity after 300 Charge-DischargeCycles Procedures T.R. = Thermal Runaway Composition of electrolytesolution (vol. %) Examples 1-4, Comparison Examples 1-5, and Examples7-10: 1M LiPF₆ + EC/PC/DEC (30/5/65, volume ratio) Examples 5 & 6: 1MLiPF₆ + EC/PC/VC/DEC (30/5/2/63/ volume ratio)

The present invention is not limited to the described examples, andvarious combinations easily reachable from the concept of the inventioncan be embodied. Particularly, the invention is not limited to thecombination of solvents described in the above-mentioned examples. Theabove-described examples are concerned with the 18650 size cylindricalbattery. However, the invention is applicable to batteries of theprismatic-type, aluminum-laminated type, and coin-type.

INDUSTRIAL UTILITY

The lithium secondary battery of the invention shows good safety forpreventing overcharging and further shows good battery characteristicssuch as cycle characteristics, battery capacity and storage stability.

1. (canceled)
 2. A lithium secondary battery comprising: a positiveelectrode comprising a compound metal oxide comprising lithium andcobalt or nickel; a negative electrode comprising a lithium metal, alithium alloy or a material capable occluding and releasing lithium; anda non-aqueous electrolyte solution comprising an electrolyte in anon-aqueous solvent, in which the non-aqueous electrolyte solutioncontains an organic compound in an amount of 0.1 to 10 weight %, saidorganic compound being a combination of a tert-alkylbenzene derivativeand cyclohexylbenzene.
 3. The lithium secondary battery of claim 2,wherein the tert-alkylbenzene derivative is selected from the groupconsisting of tert-pentylbenzene, 1-methyl-4-tert-pentylbenzene,5-tert-pentyl-m-xylene, 1-ethyl-1-(methylpropyl)benzene,(1,1-di-ethylpropyl)benzene, 1,3-di-tert-pentylbenzene, and1,4-di-tert-pentylbenzene.
 4. The lithium secondary battery of claim 2,wherein the non-aqueous electrolyte solution contains the organiccompound in an amount of 1 to 5 weight %.
 5. The lithium secondarybattery of claim 2, wherein the non-aqueous solvent comprises acombination of a cyclic carbonate and a linear carbonate.
 6. The lithiumsecondary battery of claim 2, wherein the non-aqueous solvent comprisesa combination of three cyclic carbonates and a linear carbonate.
 7. Thelithium secondary battery of claim 5, wherein the cyclic carbonate isselected from the group consisting of ethylene carbonate, propylenecarbonate, butylene carbonate and vinylene carbonate, and the linearcarbonate is selected from the group consisting of dimethyl carbonate,methyl ethyl carbonate and diethyl carbonate.
 8. The lithium secondarybattery of claim 6, wherein the cyclic carbonates are selected from thegroup consisting of ethylene carbonate, propylene carbonate, butylenecarbonate and vinylene carbonate, and the linear carbonate is selectedfrom the group consisting of dimethyl carbonate, methyl ethyl carbonateand diethyl carbonate.
 9. The lithium secondary battery of claim 2,wherein the negative electrode comprises carbonaceous material having agraphite crystal structure in which d₀₀₂ is in the range of 0.335 to0.340 nm.
 10. The lithium secondary battery of claim 2, wherein theelectrolyte is selected from the group consisting of LiPF₆, LiBF₄,LiClO₄, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiC(SO₂CF₃)₃, LiPF₄(CF₃₎ ₂,LiPF₃(C₂F₅)₃, LiPF₃(CF₃₎ ₃, LiPF₃(iso-C₃F₇)₃, and LiPF₅(iso-C₃F₇).